1. Field of the Invention
The invention relates to a preparation for making inorganic building materials water-repellent, before or after they are shaped.
2. Description of the Prior Art
The concept of making inorganic building materials water-repellent includes the prevention or reduction of water absorption, as well as the lowering of the surface tension of the building material so that its surface can no longer be wetted by water. Within the scope of this invention, the concept of building materials is understood to include materials which are set by water or carbon dioxide, such as, mortar, plaster, cement; as well as natural, shaped or set building materials or building materials consolidated by baking, such as, sandstone, tiles, pumice stone, molded concrete articles, or walling formed from these. The magnitude of the contact angle of a drop of water is frequently taken to be a measure of the effectiveness of such preparations, see Chemie und Technologie der Silicone (Chemistry and Technology of Silicones), by W. Noll, published by Chemie, 1968, page 387. This contact angle, however, provides no information regarding the amount of water which the treated building material is able to absorb or has absorbed, when it is in equilibrium with a water or water-vapor-containing environment.
In practice, the amount of water absorbed by a building material cannot be determined exactly because it depends on external conditions and on the state of the building material. It is, however, possible to measure the water absorption of a building material under standardized conditions, for example, from its increase in weight, and to obtain in this way a relative measure of the effectiveness of the water-repellent preparation. The purpose is to achieve an effective lowering of the surface tension of the treated building material and as deep a penetration as possible of the impregnating material into the building material and therefore, a lowering of the water absorption by the building material.
For many years, silicone resins have been successfully used for making building materials, such as, sandstone, lime sandstone, mortar, tiles and concrete, water-repellent. The silicone resins are obtained in a known manner from the corresponding silanes through hydrolysis or alcoholysis and condensation. Moreover, the silicone resins are applied to the building material in the form of a diluted impregnating solution. As solvents, hydrocarbons, such as, for example, gasoline, are generally used. Alkyl or aryl groups are linked as the organic groups to the silicone atoms. The methyl silicone resins, which are most easily accessible, are preferred. This state of the art is revealed, inter alia, in German Offenlegungsschrift No. 20 23 976, German Offenlegungsschrift No. 23 56 142 and German Auslegeschrift No. 25 58 184. The use of alkyl radicals with longer hydrocarbon chains has already been recommended for increasing the alkali stability, see "Bautenschutz und Bausanierung," 1980, pages 145 to 147.
The impregnating material must penetrate as deeply as possible into the capillary system of the building material in order to achieve effective water-repellency. However, this ability to penetrate decreases with increasing molecular weight of the silicone resins. Moreover, high molecular silicone resins are not suitable when very dense building materials, such as, for example, concrete, are to be made water-repellent.
German Auslegeschrift No. 20 29 446 also describes the use of organosilanes for directly impregnating walling. Admittedly, these silanes penetrate deeply into the building material. However, they have relatively high vapor pressures, so that they frequently evaporate on the surface of the material before they can condense and/or react with the inorganic building material.
Low molecular siloxanes, when used for impregnating building materials, are washed off from the surface relatively easily, as a result of which, the water repellency achieved is unsatisfactory.
For improving the surface barrier and achieving a high water-beading effect, the addition of fillers, such as, for example, silica with a specific surface area of more than 50 m.sup.2 /g, has been described in German Offenlegungsschrift No. 25 58 184. However, this filler is also worn away or washed off in time, so that the effect it produces eventually disappears.
Those skilled in the art are therefore still faced with the problem of finding a preparation which, on the one hand, penetrates deeply into the building material, and, on the other, produces a water-beading effect on the surface which is effective for a long time. In addition, a problem arises due to the susceptibility of the siloxanes to hydrolyze on alkaline media, such as, for example, concrete and fresh mortar.